Eye Astronomy #8: Why Stars Look Like That
by Dale E. Lehman
Spend a little time looking at the stars, and you’ll notice they aren’t all the same. Some are brighter than others. (Duh.) Most appear white, but some have color. Some twinkle a lot, some only a little. What causes these differences?
Distance, for one thing. Even if every star had the same intrinsic luminosity—that is, if they all emitted the same amount of light—the closest stars would be the brightest. But stars come in a wide range of intrinsic luminosities as well as distances:
Sirius, the brightest star in the sky, lies just 8.6 light years away and radiates 25 times as much light as our sun.
Rigel, another very bright star, is 864 light years distant, 100 times the distance to Sirius. But it radiates 120,000 times as much light as our sun!
The closest star to Earth, Proxima Centauri, is way dim, invisible to the naked eye. It lurks a mere 4.2 light years off but is 1,000 times dimmer than the sun.
That’s why some stars appear brighter than others. It’s the combination of luminosity and distance. Close and bright makes Sirius top dog. (Pun intended. Sirius is also known as “the dog star” because it lies in the constellation Canis Major). Close and dim hide Proxima Centauri from our eyes. And distant but extremely bright makes Rigel a stand-out star, though it doesn’t hold a candle to Sirius.
Stars also differ in color: red, orange, white. In part, that’s because of their temperatures. Just as a yellow flame is cooler than a blue flame, cooler stars appear red or orange, while the hottest stars look white or even blue.
But a second, possibly surprising factor plays into star color. In the dark, the human eye loses much of its color sensitivity. That’s why so many things appear grayish at night. This loss of sensitivity is selective. In the dark, we detect colors on the red end of the spectrum better than those on the blue end. Some stars, like Betelgeuse or Regulus, are obviously red. Arcturus (not shown in the chart) is clearly orange. But we don’t see much difference between yellow, white, and blue stars. Capella is the brightest yellow star in the sky, while Rigel is the brightest blue one, yet most people don’t notice much color difference between them.
Another terrestrial factor affects how we see the stars. After travelling tens, hundreds, or even thousands of years, the starlight strikes our atmosphere and burrows through it in just a third of a second. Short as it may be, that third of a second alters a star’s appearance. Molecules in the atmosphere scatter light, causing the star to twinkle. Some scatters so much it never reaches our eyes, and the star is dimmed. (Astronomers call this effect atmospheric extinction.) Light can even be displaced a little, altering the star’s apparent position in the sky.
Most of these effects are too small to bother the casual astronomer, but one is noteworthy: the more air light passes through, the more a star twinkles. Stars low in the sky twinkle a lot, because their light passed through a thicker slice of air than stars up high, which may twinkle only a little. At the telescope, it’s best to observe objects when they’re high up, if possible.
The degree of twinkling high in the sky is an indicator of how disturbed the atmosphere is. Astronomers call that seeing. When a calm atmosphere produces little twinkling up there, we have good seeing. A turbulent atmosphere causes a lot of twinkling and is dubbed poor seeing. Poor seeing results in blurred or distorted images in telescopes.
But sometimes poor seeing can have a fun effect. For those of us at mid-northern latitudes, Sirius is never too high in the sky. Because its light passes through so much air, it always twinkles dramatically. Here’s the cool bit. When seeing is poor, Sirius flashes through a barrage of varying colors, its brilliant light twisted by atmospheric turbulence into an ever-changing rainbow.
Such light shows aside, atmospheric interference is why so many observatories are built at high altitudes: to get them above as much of the atmosphere as practical. That’s also why space-based telescopes provide such crisp imagery. They eliminate atmospheric effects entirely. Ground-based telescopes can achieve remarkable clarity with adaptive optics, but that doesn’t correct for all atmospheric distortion.
For us living at lower altitudes, though, the universe still puts on quite a show. When we look up at the stars with just our eyes, we can marvel not only at what we see but at the incredible array of circumstances—from the nature of stars to how our own eyes work—that combine to create our sense of awe as we stand beneath the night sky.
